1. FIELD OF THE INVENTION
This invention relates to earth boring drill bits of the roller cutter type and more particularly to improvements in seal means and in bearing construction therefor to confine a lubricant in the bearing area interiorly of the cutter and to keep formation cuttings and other detritus out of such bearing area.
2. DESCRIPTION OF THE PRIOR ART
During drilling operation, the seal of a roller cutter earth boring bit encounters fluctuations in position and fluid pressure since the drill bit operates at various depths in a well in which drilling fluid is employed, and the cutter movement is complex relative to its bearing shaft in that it includes rapid axial and radial as well as wobbling motions, the magnitudes of which change as the parts are worn. Thus, a drill bit seal should operate under various conditions of operation and wear if it is to be effective to protect the bearings and enhance the useful drilling life of the bit. Also, it is desired that the seal take up as little axial space as practical so that adequate bearing capacity remains.
The prior art teaches primarily the use of rubber or other elastomeric seals in the form of an "0" ring, or the seal might be approximately square in cross-section. Packing type seals usually are solid in cross-section and are single piece seals of a single material, unlike lip seals or seals made of more then one material.
Seal manufactures have recommended that for rotary applications, an "0" ring seal, for example, should be compressed in cross-section less than 10% to prevent undue compression set, fatigue, wear and heating of the seal. For rock bit applications, the clearances and the movements between the parts may be quite large so that the seal in order to be effective must be compressed sufficiently to be able to bridge such clearances between the cutter and its shaft as may occur. This may indicate that the seal should be compressed in cross-section more than 10% because for an "0" ring having a cross-sectional diameter of 0.139 inches, a 10% squeeze would be only about 0.014 inches whereas the cutter movement toward and away from its shaft may be substantially more than 0.014 inches. However, simply to compress the seal more than 10%, which would solve the problem of greater amounts of movement of the parts, may result in compression set and other seal problems as previously stated.
It would seem that a solution to the problem would be to use an "0" ring having a substantially larger cross-sectional diameter such that the percent compression would be small while the total compression of the ring would be sufficient to take care of the expected cutter movements. However, in rock bits the space available is very limited and the use of large "0" rings would result in less room for the bearings, so this would not be a practical solution. Murdoch, et al U.S. Pat. No. 3,765,495 discloses the use of a radially elongated seal in hoop compression which gives the improved sealing capacity needed in a limited space.